1. Field of the Invention
The present invention relates to a method and an apparatus for receiving digital television signals. More particularly, the invention relates to a digital television signal receiving apparatus in which optimal automatic gain control (AGC) points are individually set for a terrestrial digital television signal and a cable television (CATV) digital signal by switching the AGC points according to the type of input digital television signal. The invention also pertains to a digital television signal receiving method for use in the above type of apparatus.
2. Description of the Related Art
Digital television signal receiving apparatuses are coming into wide and general use. A tuner of a digital television signal receiving apparatus is provided with AGC circuits for detecting changes of the amplitude of an input signal and for keeping the amplitude of an output signal constant.
The AGC circuits for use in the digital television signal receiving apparatus are grouped into two types: one type is an AGC circuit for a radio frequency (RF) signal receiving apparatus, and the other type is an AGC circuit for an intermediate frequency (IF) signal receiving apparatus. In the RF AGC circuit, the gain of an RF signal received from an antenna is controlled. In the IF AGC circuit, the gain of an IF signal converted from an RF signal is controlled.
Normally, the IF AGC operation is first conducted, and when the input level of the tuner becomes constant, the RF AGC operation is conducted. The reason for this is as follows. The IF signal is first controlled so that an image with a good signal-to-noise (SIN) or carrier-to-noise (C/N) ratio can be obtained with the highest level of sensitivity of the tuner. Then, the RF signal is controlled so that the gain of the tuner is suppressed so as to prevent cross modulation distortions.
FIG. 1 illustrates a conventional AGC circuit for controlling an RF AGC point. A reference voltage V0 output from a reference power supply source 2 is input into an inverting input terminal of a comparator 1 shown in FIG. 1, while an IF AGC signal is input into a non-inverting input terminal of the comparator 1 from a circuit (not shown) subsequent to this AGC circuit.
The comparator 1 then compares the IF AGC signal input into the non-inverting input terminal with the reference voltage V0 input from the reference power supply source 2. Upon comparison, as shown in FIG. 2, if the voltage of the IF AGC signal is equal to or higher than the reference voltage V0, an RF AGC signal with a constant high level is output. If the voltage of the IF AGC signal is lower than the reference voltage V0, an RF AGC signal with a gradually reduced voltage level is output.
When the output voltage of the RF AGC signal from the comparator 1 starts to be reduced, a tuner (not shown) assumes that an IF signal having an adequate magnitude has been obtained in a circuit (not shown) subsequent to the comparator 1 and begins to control the level of the RF signal to be output. That is, when the voltage of the IF AGC signal reaches the reference voltage V0, the AGC point is set (the point at which the RF AGC operation is performed).
Generally, concerning terrestrial signals, although the interruption level may become higher than that of a desired signal, the noise level is much the same as the thermal noise level. In contrast, regarding the CATV signals, the interruption level is not very high, but the noise level is higher than the thermal noise level.
When, for example, the bandwidth is 6 MHz, the thermal noise level is xe2x88x92106 dBm. When the noise figure (NF) of the tuner is 7 dB, and the C/N ratio required for an output of the tuner is 25 dB, the AGC (RF AGC) point of the tuner when terrestrial signals are received is calculated according to equation (1) below. In calculating the AGC points, the S/N ratio and the C/N ratio are treated equally considering that they are the ratios of the input level of terrestrial signals and CATV signals, respectively, to the noise level.
xe2x88x92106+7+25=xe2x88x9274 dBmxe2x80x83xe2x80x83(1)
In setting the AGC point of the tuner when receiving CATV RF signals, when, for example, the input RF signal level is xe2x88x9255 dBm and the corresponding C/N ratio is 27 dB, the noise level becomes xe2x88x9282(=xe2x88x9255xe2x88x9227) dBm. Since the level of the input RF signal is lower than the AGC point expressed by equation (1), i.e., xe2x88x9274 dBm, the AGC operation is conducted on the input RF signal by an amount of gain, i.e., 19(=xe2x88x9255xe2x88x92(xe2x88x9274)) dB, by which the input level is reduced from the AGC point. However, an AGC operation is performed also on the NF, and the NF results in 26 (=19+7) dB. In this case, the output of the tuner C/N(TUNER OUT) can be calculated by equation (2) below:
C/N(TUNER OUT)=10Log(Ci/(Ni+(NFxe2x88x921)kTB))xe2x80x83xe2x80x83(2)
where Ci indicates the level of the input RF signal (true value) of the tuner; Ni represents the input noise level (true value) of the tuner; k is the Boltzmann constant (=1.38xc3x9710xe2x88x9223 JKxe2x88x921); T is the absolute temperature (300 K); and B represents the bandwidth (6 MHz). Ci, Ni, and NF can be determined by the following equations.
ci=10((xe2x88x9255xe2x88x9230)/10)≈3.2xc3x9710xe2x88x929 xe2x80x83xe2x80x83(3)
Ni=10((xe2x88x9282xe2x88x9230)/10)6.3xc3x9710xe2x88x9212 xe2x80x83xe2x80x83(4)
NF=10(26/10)≈400 xe2x80x83xe2x80x83(5)
By substituting the above-described values and the calculation results of equations (3) through (5) into equation (2), the following equation (6) can be obtained.
C/N(TUNER OUT)=10Log((3.2xc3x9710xe2x88x929)/((6.3xc3x9710xe2x88x9212)+(400xe2x88x921)xc3x97(1.38xc3x971031 23)xc3x97300xc3x97(6xc3x97106)))=10Log(197.5)≈23 (dB)xe2x80x83xe2x80x83(6)
The resulting value obtained by equation (6) is smaller than the output C/N=25 dB required for the tuner by 2 dB.
Thus, as stated above, in receiving the terrestrial RF signal, it is necessary to increase the AGC point to obtain a higher sensitivity in order to reduce cross modulation distortions. In other words, the AGC point must be set so that the AGC circuit is operated even if the input RF signal is reduced only by a smaller amount. In contrast, in receiving the CATV RF signal, since the noise level thereof is higher than that of the terrestrial signal, it is necessary to reduce the AGC point to obtain a lower sensitivity compared to that of the terrestrial signal in order to receive the RF signal with a good C/N ratio. In other words, the AGC point must be set so that the AGC circuit is operated when the input RF signal is reduced by a greater level. It is thus difficult to individually set optimal AGC points for the above two types of signals, i.e., the terrestrial signal and the CATV signal.
Accordingly, in view of the above background, it is an object of the present invention to individually set optimal AGC points for a terrestrial digital television signal and a CATV digital signal.
In order to achieve the above object, according to one aspect of the present invention, there is provided a digital signal receiving apparatus for receiving a first signal and a second signal. The digital signal receiving apparatus includes a setting unit for individually setting an AGC point for the first signal and an AGC point for the second signal, and a switching unit for switching between the AGC points set by the setting unit according to whether an input signal is the first signal or the second signal.
According to another aspect of the present invention, there is provided a digital signal receiving apparatus for receiving a digital terrestrial broadcast and a digital cable television broadcast. The digital signal receiving apparatus includes a tuner provided with a first AGC circuit controlled by a first control signal. The tuner converts an RF signal of one of the digital terrestrial broadcast and the digital CATV broadcast into an IF signal and outputs the IF signal. A second AGC circuit controls the level of the IF signal supplied from the tuner based on a second control signal. A processing circuit processes an output signal of the second AGC circuit and also generates the second control signal. A control circuit controls the level of the first control signal based on a result of comparison between the second control signal and a reference signal. A switching circuit switches the level of the reference signal according to whether the digital terrestrial broadcast or the digital CATV broadcast is received.
According to still another aspect of the present invention, there is provided a digital signal receiving method for use in a digital signal receiving apparatus for receiving a first signal and a second signal. The digital signal receiving method includes a setting step of individually setting an AGC point for the first signal and an AGC point for the second signal, and a switching step of switching between the AGC points set in the setting step according to whether an input signal is the first signal or the second signal.
With this arrangement, the AGC points are individually set for the first signal and the second signal and are switched according to whether an input signal is the first signal or the second signal. It is thus possible to set optimal AGC points for the first signal and the second signal.